When a sip at sea suffers a collision resulting in a penetration of its hull below the waterline, repairs must normally be made before the ship's voyage may be continued. The continued safety of the vessel is often dependent on stopping the inflow of water resulting from the breach.
While the ship's pumps are used to lessen the effects of flooding, temporary repairs to stop or reduce the leakage can sometimes be made from within the vessel. However, more permanent external repairs are preferred because they offer better structural integrity for the ship than do internal repairs.
The most common form of temporary external repair is effected by welding a steel patch over the breach on the outside of the hull. This type of repair is preferred because the hydrostatic pressure of water against the patch causes little or no additional stress on the welds holding the external patch in place.
Two problems are associated with external repairs made below a ship's waterline. First, the ship must be stopped to permit the repair to be made. This delays the ship's arrival at its port of destination. Second, wave action combined with the ship's rolling motions makes the task of positioning and welding a patch to the hull of a ship, underwater very hazardous to the divers making the repairs. As a result, nearly no external repairs to underwater hull penetrations are currently made on commercial vessels while afloat. Those repairs that are effected are done with great care, considerable danger, are very temporary and generally take a long time. In lieu of such repairs, the ship is normally taken to the nearest available dry dock under its own power, or under tow if necessary, where the needed repairs are carried out.
At any given time, only a small number of available dry docks of the necessary size are normally available. This results in long detours from the ship's planned route to and from the dry dock. The time required for such transit plus the time needed for repairs can result in the ship being out of service for a considerable period. As a result, collision repair is extremely costly to the operation of a vessel. A need therefore exists for a safe means of repairing a ship at sea. If a sufficiently sound patch could be safely placed and welded over an underwater penetration of a ship's hull, the ship would then be able to continue its planned itinerary and obtain permanent repairs in dry dock when and where available.